Groovy++ pattern for listeners

gistfile1.txt

  abstract static class Listener<T> {
    abstract void call(T oldValue, T newValue)
  }

  private FList<Listener<T>> listeners = FList.emptyList

  final Listener<T> addListener(Listener<T> listener, Executor executor = null) {
    if(executor) {
      def myListener = listener
      listener = { oldValue, newValue ->
        executor.execute {
          myListener(oldValue, newValue)
        }
      }
    }

    for(;;) {
      def old = listeners
      if(listeners.compareAndSet(old, old + listener))
        break
    }
  }